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Crops will not grow in climates that are too hot, too cold or too dry. Temperature and moisture are thus the two fundamental factors that govern crop production. In conjunction with solar radiation, they determine a crop's potential yield.
The first basis of the AEZ climatic inventory is a division of land in the developing countries into 14 major thermal regimes, determined by the average temperature during the period when there is sufficient moisture for crop growth. The temperature divisions were chosen to correspond with the temperature needs of the crops under study.
Calculating length of growing period
In terms of major climates alone, areas where monthly mean temperature at sea level is greater than 18°C in every month were designated tropical; those where it falls below 18°C but remains above 5°C were designated subtropical; and those where it falls below 5°C in any month were designated temperate. The subtropical areas were further divided into those with summer or winter rainfall. All these areas, were then divided into warm, cool and cold, making 14 thermal regimes (see page 7).
1: Length of growing periods - Africa
Providing temperature requirements are met, a crop's potential yield is determined by how well its growth cycle meshes with the period when sufficient moisture is available for growth. Length of growing period, the number of days in which temperature and water permit crop growth, was therefore used as the other major factor determining crop growth. This was a new approach.
The AEZ study defined the growing period to include two parts. First the number of days when precipitation exceeds half the potential water loss from evaporation and plant transpiration, a condition which allows water to build up in the soil. This period is then extended to include a further period during which 10 cm of water assumed to be stored in the soil are evaporated or transpired. Various amounts of water storage can of course be included in the model.
Finally, the lengths of growing period are adjusted to allow for periods when mean temperature falls below the minimum needed for crop growth - about 6.5°C in the case of wheat in North Africa, for example. Thus, although water may be available for crop growth for 150 days, if the temperature is too low during 30 of those days, the length of growing period is defined as 120 days.
STEP 2a
2: Major climates and thermal regimes
The climatic inventory is assembled by overlaying the map of the major climates and thermal regimes with a map showing length of growing periods. This overlay is necessary in order to compute the extent of each agro-ecological cell at a later stage in the study (see pages 12/13).
The growing period begins with the start of the rainy period and ends some time after it stops, when an assumed 10 cm of stored moisture (if available) in the soil have been used up. The methodology also takes account of irregularities in both rainfall and temperature during the period.
The climatic inventory (3) is prepared by overlaying the map showing length-of-growing-period zones (1) on the map of the major climates and thermal regimes (2). The overlay, shown here for Africa only, is carried out for each of the developing regions.
Only six aggregated length-of-growing-period zones are included in the map shown here. In the AEZ study, the zones were eventually drawn at 30-day intervals; in drier areas a zone of 75 to 90 days was drawn to accommodate moisture requirements for millet and sorghum production. In total, 21 different growing-period zones were distinguished.
The working scale was 1:5 million, fifteen times larger than the simplified maps reproduced here.
